The present disclosure relates to a non-aqueous electrolyte secondary battery, and in detail, to a non-aqueous electrolyte secondary battery containing a carbonaceous material as a negative active material capable of occluding and releasing an electrode reaction substance.
According to recent trends in cordless and portable features of electronic instruments such as VTR (video tape recorder) with integrated camera, mobile phone, notebook-type personal computer and so forth, there have been active efforts of developing thin, small and lightweight portable electronic instruments one after another. Also with advancement in wider functionality and higher performance of the instruments, power consumption of the portable electronic instruments has been increasing, so that there are growing needs for larger capacity and reduction in weight of batteries as an energy source of these electronic instruments. In particular, secondary battery is understood as a key device for these electronic instruments, and is now a target of active research and development aimed at improving the energy density, and consequently non-aqueous electrolyte secondary battery using a non-aqueous electrolyte as the electrolyte has already been launched. Among others, lithium ion secondary battery, making use of doping/undoping, or inserting/extracting, of lithium ion, has widely been used by virtue of its high energy density as compared with conventional aqueous solution-base batteries such as lead battery, nickel-cadmium battery and so forth.
Conventional lithium ion secondary battery makes use of lithium cobalt oxide for the positive electrode, and a carbonaceous material for the negative electrode, wherein charging in a non-aqueous electrolyte results in electrochemical doping of lithium in the positive electrode into interlayer of carbon composing the negative electrode. Carbon thus doped or having inserted with lithium can act as a lithium electrode, while allowing lithium to be undoped or extracted from the carbon interlayer with progress of discharging, and to go back into the positive electrode. With regard to such lithium ion secondary battery, Japanese Patent Application Publication (KOKAI) No. H03-252053 proposed a method of improving the cycle performance, by using a non-graphitizable carbonaceous material, having an interplanar spacing of the (002) surfaces of 3.70 Å or larger, and a true density of smaller than 1.70 g/cm3 as the negative electrode, so as to improve acceptability of lithium ion into the carbon interlayer of the negative electrode.
In these days, graphites which are high-crystallinity carbonaceous materials having well-developed crystal structures have been used more widely as a negative electrode material. The graphites have true density larger than that of non-graphitizable carbonaceous materials having only low crystallinity, and can therefore raise the electrode packing performance when they are used as materials for the negative electrode.
As a countermeasure for liquid leakage which may otherwise be bothersome when a liquid-base electrolyte was used, lithium ion polymer secondary batteries, for example, using a gel-form polymer film impregnated with a non-aqueous electrolyte solution, or a solid-state electrolyte as the electrolyte, have been put into practical use. This sort of lithium ion polymer secondary batteries are characterized by having a self-supporting property of the battery element per se, because the electrolyte per se is immobilized, and further because the interface between the electrode and the electrolyte is immobilized.
Since these lithium ion polymer secondary batteries can use a laminated film for packaging, they can be thinned by deep drawing. They can also be reduced in weight as compared with those packaged by metal. They are appropriately adoptable to power sources for compact mobile instruments, because they are composed of a smaller number of components as compared with conventional batteries using battery can, and can therefore be manufactured at low costs.
This sort of non-aqueous electrolyte secondary batteries require shortening of charge time for improved convenience. Related art non-aqueous electrolyte secondary batteries, however, suffer from a problem in that lithium deposits onto the surface of the negative electrode with progress of the charge-discharge cycles, when they undergo repetitive charging under a high load rate (high-load charging) aimed at shortening the charge time. Deposition of lithium may decrease the amount of lithium ion insertable into the negative electrode, and may gradually decrease the capacity with progress of the charge-discharge cycles. Moreover, thus-deposited dendritic lithium may injure the separator, which is causative of micro-shortcircuiting, and may degrade the cycle performance. The dendritic lithium may even grow to penetrate the separator, and may cause internal shortcircuiting. It is therefore necessary to suppress the lithium deposition under high-load charging, and to improve the cycle performance.